The present invention relates to a pneumatic tire, more particularly to a tread pattern capable of improving mud performance without sacrificing noise performance and steering stability and thus suitable for sport-utility vehicles, pickup trucks and the like.
Usually, four-wheel-drive vehicles for use on paved or public roads as well as off paved roads, e.g. sport-utility vehicles, pickup trucks and the like, are provided with pneumatic tires having block-type tread patterns made up of a large number of blocks separated from each other by tread grooves to improve running performance on muddy roads (mud performance).
In order to improve the mud performance, a conventionally employed technique is to increase the percentage of the grooved area of the block-type tread pattern for example by increasing the widths of axial grooves.
However, if the percentage of the grooved area is increased, noise performance during running on paved roads is deteriorated. Further, as the rigidity of the blocks are decreased, the steering stability during running on paved roads is deteriorated. Thus, the mud performance and noise performance and the mud performance and steering stability are antinomic, and it has been believed to be difficult to improve these performances up to satisfactory levels.
The present inventor therefore, made a study on effects of circumferential and axial grooves on the mud performance. As a results, it was discovered that axial grooves do not function well to improve the traction on muddy roads because mud penetrating in the axial grooves remains undischarged during running, and the traction is produced mainly between the mud in the axial grooves and mud on the road surface, and that the axial grooves do not contribute to the improvement in the mud performance contrary to expectation. In the case of the circumferential groove, on the other hand, the mud therein can be discharged during running (or self-ejected) easier than the axial grooves. In the case of a straight circumferential groove, the mud within the groove slides in the groove and can not produce substantial traction. In the case of a zigzag circumferential groove, the mud within the groove is hard to slide in the groove and a shearing force from the mud on the road surface occurs to produce traction. As to the self-ejection of mud, the circumferential groove is superior to the axial grooves. As the circumferential groove is continuous in the circumferential direction, it can produce a relatively large shearing force continuously and stably during running.
In the case of the axial grooves, the penetration of the mud into the axial grooves is only from bottom up. But, in the case of the circumferential groove, the mud also enters from the front and from behind. In other words, the circumferential groove allows the mud to penetrate into the groove quickly, therefore, the mud can produce traction without delay.
As explained above, as to the degree of contribution to the traction, a circumferential groove is higher than axial grooves. Accordingly, to increase the shearing force by improving a zigzag configuration and a width of a circumferential groove is most effective for improving the mud performance. However, in order to effectively derive the function to increase the traction from the circumferential groove, it is necessary to even the ground pressure distribution, and the number of the axial grooves is an important factor therefor. If the number of the axial grooves is insufficient, then during running, the ground pressure measured in the vicinity of the circumferential groove becomes increased from one axial groove to the next axial groove, namely, between the axial grooves. As a result, the mud in this part of the circumferential groove between the axial grooves, becomes hard to be trodden down compactly, and the shearing force in this part decreases. Accordingly, the traction is decreased.
In order to avoid such decrease, it is necessary to even the ground pressure distribution by increasing the number of the axial grooves in the footprint of the tire. As explained, the axial grooves themselves do not increase the traction directly, therefore, it is possible to decrease the width of the axial grooves, and thereby it is possible to prevent the grooved area percentage from increasing even the number of the axial grooves is increased. As a result, the mud performance can be improved without sacrificing the noise performance and steering stability.